Chucks are used to securely hold a tool or work piece and to rotate that tool or work piece about a central axis. More particularly, a chuck includes a plurality of jaws, each of which is movable in a radial direction relative to the rotational axis of the chuck. Typically, the chuck is securely mounted to a rotationally drivable member such as a drill or machine tool. With the machine tool in a non-rotating mode, the chuck will be activated to cause the jaws thereof to move in a radially outward direction, thereby enabling a tool or work piece to be inserted therebetween. The jaws of the chuck then are moved radially inwardly to securely grasp the tool or work piece. After the work piece, tool or the like has been securely grasped, the machine tool or drill to which the chuck is mounted can be activated, thereby causing the chuck and the work piece or tool mounted therein to rotate. This rotation enables a tool mounted in the chuck to perform work, or enables a work piece mounted therein to be appropriately machined.
Many available chucks enable a tool or work piece to be securely held at relatively low speeds. However, this low speed operation adds proportionally to the amount of time required to complete a work task. Furthermore, the low speed operation often will not generate sufficient forces to perform work on many metallic materials, and low speed operations can negatively affect the quality or smoothness of the machined surface. In many applications, such as machining of aluminum disks for video cassette recorders, a very high degree of precision is required. For example, many such disks must be accurate to within one micron of concentricity on a disk having a diameter of between three inches and five inches. This degree of precision generally is not attainable with prior art rotating machine tools that operate at low speeds. Consequently it is necessary to carry out a separate polishing or grinding step to achieve the required precision. This, of course, is quite costly, and further adds to the time required to manufacture the article.
Ceramic cutting tools and bits are available which enable a very precisely finished cut at high speeds. These ceramic cutting tools and bits are theoretically capable of providing the desired degree of accuracy without a subsequent polishing step. However, the ability of these ceramic bits to cut accurately depends upon the ability of the chuck to securely and accurately retain the rotating tool or work piece. As the chucks are rotated with increasingly rapid speed, various members of the chuck are distorted outwardly by centrifugal forces. The magnitude of these centrifugal forces increases with increasing rotational speeds. The effect of these forces is to loosen the holding power of the chuck. As a result, the rotating tool or work piece is held less securely and nonconcentric imprecise cutting results. Consequently, when the ceramic bits are used in the prior art chucks, the machining time is reduced, but accuracy is not improved appreciably. As a result, subsequent polishing operations often are required.
In addition to the loosening caused by distortion of prior art chuck members rotating at high speeds, a certain amount of loosening in the prior art chuck is caused by relative movements between chuck components. More particularly, the prior art chuck generally includes a central actuating member that moves in an axial direction in response to fluid pressure. The actuating member includes inclined channels that engage the master jaws of the chuck. The inclination of the jaw channels in the actuating members is such that as the actuating member moves rearwardly and away from the work piece, the master jaws slide generally radially inwardly to grip the tool or work piece. However, this inward motion is not purely radial. Rather, each master jaw will rotate slightly forward or toward the work piece. This rotation is due to the small but inevitable looseness of the master jaw in the channel of the actuator. Rotation of the chuck creates radially outward centrifugal forces which cause the master jaw to rotate into a more nearly radial alignment. This rearward rotation of the master jaw effectively loosens the grip on the work piece. Since the prior art chuck achieves its holding power due to the rearward forces on the actuator, there is no force to compensate for this rotation of the master jaw.
Many chucks have been developed which attempt to offset the effects of centrifugal forces at high speeds. The most typical structure for offsetting the effects of centrifugal forces includes an array of counterweights. The counterweights are pivotally mounted in the chuck such that at high speeds the heavier end of the counterweight is urged radially outwardly by centrifugal forces, while an opposing portion of the counterweight is rotated into the jaw to offset the effect of centrifugal forces. Examples of such chucks are shown in: U.S. Pat. No. 3,984,114 which issued to Ovanin on Oct. 5, 1976; U.S. Pat. No. 4,009,888 which issued to Wallace on Mar. 1, 1977; and U.S. Pat. No. 4,431,201 which issued to Morisaki on Feb. 14, 1984. A similar arrangement with counterweights acting on the central driving member of the chuck jaws is shown in U.S. Pat. No. 3,467,404 which issued to Sloan on Sept. 16, 1969. Still other devices employing springs, wedges and pawls to offset centrifugal forces are shown in U.S. Pat. No. 4,213,621 which issued to Fink et al on July 22, 1980; U. S. Pat. No. 4,213,623 which issued to Rohm on July 22, 1980; U.S. Pat. No. 4,437,675 which issued to Koenig on Mar. 20, 1984; U.S. Pat. No. 4,139,206 which issued to Knohl on Feb. 13, 1979; U. S. Pat. No. 4,139,207 which issued to Grimes on Feb. 13, 1979 and U.S. Pat. No. 4,206,932 which issued to Felker on June 10, 1980.
It is been found that these prior art chucks provide acceptable control of centrifugal forces at fairly low rotational speeds. At higher speeds, however, the prior art chucks are susceptable of being distorted with a resulting decrease in holding power and accuracy of the work performed.
Many machine tool operaters have attempted to overcome the effects of centrifugal force by merely applying a higher chucking pressure. The theory is that a reduction in the radially inward holding force caused by the opposing centrifugal forces can be accepted if the initial holding forces are high. This theory may be acceptable in certain applications. However, in many applications this higher initial holding force may damage the article being held in the chuck.
It also has been found that many chucks that deform during high speed rotation will effectively elastically rebound at the termination of the high speed rotation. This elastic rebounding will cause the jaws to create a brief radially inward residual force on the work piece that exceeds the initial force exerted by the jaws. These residual forces are even more likely to damage the work piece then the initial forces exerted by the jaws.
In addition to the above described problems with high speed chucks, it is known that lubrication in high speed chucks is especially important. Automated lubricating mechanisms have been built into chucks. However, these have tended to be extremely complicated, inefficient and/or unreliable. Consequently many machine tool operators have relied upon frequent manual lubrication with even the most sophisticated chucks. Obviously, this manual lubrication is inefficient and time consuming.
In view of the above, it is an object of the subject invention to provide an improved chuck for high speed operations.
It is a further object of the subject invention to provide a simple chuck that is capable of securely retaining a tool or work piece at extremely high rotational speeds.
It is another object of the subject invention to provide a high speed chuck that does not require an unacceptably high initial griping pressure.
It is an additional of the subject invention to provide a high speed chuck that substantially avoids problems associated with elastic rebounding of the jaws at the termination of a tooling operation.
Another object of the subject invention is to provide a high speed chuck that includes a simple and reliable lubrication of the various moving parts thereof.